The present invention relates to a tip turbine engine, and more particularly to a starter-generator system.
An aircraft gas turbine engine of the conventional turbofan type generally includes a forward fan, a low pressure compressor, a middle core engine, and an aft low pressure turbine all located along a common longitudinal axis. A high pressure compressor and a high pressure turbine of the core engine are interconnected by a high spool shaft. The high pressure compressor is rotatably driven to compress air entering the core engine to a relatively high pressure. This high pressure air is then mixed with fuel in a combustor and ignited to form a high energy gas stream. The gas stream flows axially aft to rotatably drive the high pressure turbine which rotatably drives the high pressure compressor through the high spool shaft. The gas stream leaving the high pressure turbine is expanded through the low pressure turbine which rotatably drives the fan and low pressure compressor through a low pressure shaft.
Although highly efficient, conventional turbofan engines operate in an axial flow relationship. The axial flow relationship results in an elongated engine structure of considerable longitudinal length relative to the engine diameter. This elongated shape may complicate or prevent packaging of the engine into particular applications.
A recent development in gas turbine engines is the tip turbine engine. Tip turbine engines locate an axial compressor forward of a fan which includes hollow fan blades that receive airflow from the axial compressor therethrough such that the hollow fan blades operate as a centrifugal compressor. Compressed core airflow from the hollow fan blades is mixed with fuel in an annular combustor and ignited to form a high energy gas stream which drives the turbine integrated onto the tips of the hollow fan blades for rotation therewith as generally disclosed in U.S. patent Application Publication Nos.: 20030192303; 20030192304; and 20040025490.
The tip turbine engine provides a thrust to weight ratio equivalent to conventional turbofan engines of the same class within a package of significantly shorter length.
Conventionally, a dedicated starter and a dedicated generator with both machines mounted onto a gearbox that is coupled to the high spool shaft via a gear driven towershaft. As the starter accelerates the engine, a fuel delivery pump driven by a gearbox attached to a rotor of the gas turbine engine provides fuel flow thereto. Igniters are then actuated to effect ignition in a combustor of the engine. Upon successful ignition, and once the engine has reached a self-sustaining speed, the starter is disengaged and the generator is engaged.
Accordingly, it is desirable to provide a lightweight starter-generator for a tip turbine engine which avoids an accessory gearbox and operation in an oil environment.